Reflexes evoked by movement of the ankle joint, and vestibular responses elicited by natural and galvanic labyrinthine stimulation, will be investigated with regard to their role in human postural stabilization during upright stance. Subjects will stand upon a platform whose motion is under computer control. The platform enables independent, quantitative alterations of ankle rotation and antero-posterior body sway, including the fixation of these variables and the sensory inputs associated with them. Simultaneous monitoring of surface EMG in 4 leg muscles, measurement of ankle torque, and on-line determination of body position and joint angles will be used to characterize all postural responses, subject to computer-aided statistical analysis. The research plan is designed: 1. To quantify ankle reflex and vestibular responses in individual muscles under conditions of quiet stance. 2. To describe the properties of intermuscular coordination at several joints during postural stabilization. 3. To demonstrate and specify the adaptation or facilitation of postural responses under altered sensory conditions. 4. To compare response properties of neurologically normal subjects with those seen in patients with known cerebellar deficit. 5. To begin differentiation among central and spinal mechanisms responsible for postural stabilization under changing conditions.